Father Time and he's taking an 8 count right now. He's not heading to the comeback trail, he's mid-30s and has 40+ fights. This is a pretty long career for the modern era already. Power is supposed to be the last thing to go but Wilder isn't really a pure power puncher, it's mixed with speed, reflexes, and endurance and those things don't age as well. Wilder always showed up in condition, fought for a KO to the final bell, never lost to a scrub, and stayed consistently active. That's more than you can say for almost anyone at this point. As far as who I want to see it's Ruiz. The unstoppable force meets the immovable object. It would be a great test of how big a fluke his win over AJ was.
I would like to see him have the pro rematch with Evgeny Romanov. Or retire. He has been used and used up. He made a lot of money along the way and cannot complain, but there is nowhere for him to go. His entire career has proved that he struggles even with the fringiest of fringe contenders, so why go on when he has not beaten a world class opponent TO THIS DAAAAY? But Romanov would be intriguing.